1. Field of the Invention
The present invention to a CMP pad conditioner which is used for smoothing and planarizing a surface of a workpiece such as silicon wafer in a CMP apparatus.
2. Discussion of the Related Art
In recent years, as a process of smoothing and planarizing a surface of a silicon wafer or the like, there is commonly practiced a chemical mechanical polishing (herein after referred to as “CMP”) process.
FIG. 3 shows a conventional CMP apparatus 51 including: a rotary table 53 which is to be rotated about its axis by a drive shaft 52; an polishing unit 54 which is disposed above the rotary table 53; a conditioning unit 55 which is disposed above the rotary table 53; and a polishing pad 56 which is formed on an upper surface of the rotary table 53.
The polishing unit 54 includes a polishing spindle head 57 and a disk-shaped wafer carrier 58 having a lower surface to which a wafer 59 as a workpiece is to be fixed. In this example illustrated in FIG. 3, the wafer 59 is sucked by the wafer carrier 58. The sucked wafer 59 can be rotated together with the wafer carrier 58, about an axis of the disk-shaped wafer carrier 58 by the polishing spindle head 57. The conditioning unit 55 includes a conditioning spindle head 60 and a conditioning disk 61 which can be rotated about its axis by the conditioning spindle head 60.
The CMP apparatus 51 further includes a slurry supplier 62 provided to supply an abrasive slurry 63 onto the polishing pad 56. In a polishing operation, the supplied slurry 63 is caught between the wafer 59 and the polishing pad 56 which are held in contact with each other. With the wafer 59 being held contact at its surface with an upper surface of the polishing pad 56 which is disposed on the upper surface of the rotary table 53, the contact surface of the wafer 59 is ground or polished by the slurry 63.
On a radially outer portion of a lower surface of the conditioning disk 61, there are fixed abrasive grains such as diamond grains. The abrasive grains fixed to the lower surface of the conditioning disk 61 are rubbed against the surface of the polishing pad 56, for dressing and truing the surface of the polishing pad 56. Thus, the surface of the polishing pad 56 is prevented from becoming compacted, and is kept suitably roughened so as to maintain its abrasive performance.
FIGS. 4A and 4B shows a conventional conditioning disk, which is principally constituted by a disk-shaped metal substrate 71 and abrasive grains 73. The metal substrate 71 includes an annular portion 72 provided by its radially outer portion. The annular portion 72 is made to protrude in an axial direction of the substrate 71, and is given a flat end surface. The abrasive grains 73 are disposed on the flat end surface of the annular portion 72, and arranged in a predetermined pattern. However, when this conventional conditioning disk is used for conditioning a polishing pad having an elasticity, the conditioning disk suffers from a problem that ones of the abrasive grains 73 located at a peripheral part of the annular portion 72 tends to be easily worn, since a large load acts on the ones of the abrasive grains 73 located at the peripheral part. In the worst case, the abrasive grains 73 at the peripheral part could be fractured or removed from the substrate 71. If the abrasive grains 73 at the peripheral part were worn out, the polishing pad no longer could be satisfactorily dressed and trued. That is, it would be impossible to sufficiently clean the pad surface and remove high spots on the pad surface. In other words, the conditioning disk could no longer serve as a conditioner. Further, if the abrasive grains 73 were fractured or removed from the substrate 71, the fractured or removed grains left on the pad surface could cause the workpiece to be scratched, so that the polishing operation would have to be suspended.
JP-A-2001-113456 and JP-A-2001-287150 (publications of unexamined Japanese Patent Applications laid open in 2001) discloses CMP pad conditioners having respective arrangements designed for solving the above-described problems. In either of the conditioners disclosed by the two Japanese publications, the disk-shaped substrate is provided by an annular body, and has a working surface which is provided by one of its axially opposite end surfaces. The working surface is entirely or partially defined by a part of a spherical surface, so as to be convexly curved. That is, the working surface is entirely or partially provided by a convexly curved surface having a predetermined radius of curvature as measured in an axial cross section of the disk-shaped substrate. Since the working surface is entirely or partially provided by the convexly curved surface, it is possible to reduce a load acting on the abrasive grains located in a peripheral or radially outer portion of the substrate. However, since the flat or non-curved portion of the working surface is narrow, the conditioner cannot perform a conditioning operation at a high efficiency. Further, since the abrasive grains located on a relatively outer portion of the working surface of the disk-shaped substrate tend to be worn earlier than those located on a relatively inner portion of the working surface of the substrate, the pad is likely to suffer from a wear in a local portion of its surface, which could lead to a reduction in the pad lifetime. For avoiding the local wear of the pad surface, it might be possible to feed the conditioner on the pad surface along a denser tool path with an increased number of times of its reciprocating motions. However, feeing the conditioner along the denser path requires a larger length of time as a conditioning time, and is not therefore practicable.